When acquiring MR data it is relatively easy to control the slice location and the width of each of the slices. However, when a region or volume of interest is within a selected slice it is more difficult to obtain data from the region or volume of interest and exclude data from the rest of the slice. That is, in-plane localized data acquisition is difficult.
One of the methods of in-plane localization uses surface coils. If the region of interest is a relatively small area, a small coil has to be used to obtain localized data. However, a small coil limits the penetration depth and thereby precludes obtaining multiple slices. If a tumor is located in the head, for example, multiple slices would be used to look at the tumor. To focus on the tumor to the exlusion of extraneous data, a region of interest in each of the slices must be selected. Hence it is important to be able to acquire data from the region of interest in a multiplicity of slices.
There are other prior art methods for selective volume excitation. See for example, a communication in the Journal of Magnetic Resonance, Volume 70, pages 488-492 (1986) entitled "Selected Volume Excitation Using Stimulated Echoes (VEST). Applications to Spatially Localized Spectroscopy and Imaging", written by the inventor herein. In the procedure explained in the communication, a volume of interest is excited using stimulated echoes with Gx, Gy and Gz gradients.
Among the drawbacks of the prior art methods is that none of the prior art methods of selecting volumes of interest (VOI) in the subject within a selected slice are amenable to multi-slice acquisition. Also, many of the prior art methods increase dependence on T2 and/or require complicated programs or extra hardware to implement.
Another of the problems of the prior art used to provide localization of data from a selected slice obtained for MR spectroscopy or imaging are the adverse effects of eddy currents. In the prior art saturation methods are often used in an attempt to minimize the eddy current problems where spins in volumes outside the VOI are saturated and the VOI is left unsaturated. It has been found that methods that saturate volumes outside the VOI has the advantage of minimizing eddy current problems. Saturation methods are especially helpful where large volumes do not have to be saturated. For example, in head imaging or limb imaging, the saturation methods are extremely useful and advantageous.
Accordingly, there is a need for improved magnetic resonance spatial localization methods using saturization effects in obtaining magnetic resonance data of sections of interest in selected slices in a multi-slice selecting sequence for spectroscopy and for imaging.